Conventionally, an ignition device is known which merges a spark plug and an ignition coil in one housing as disclosed in U.S. Pat. No. 6,694,958-B and its counterpart JP-2003-297654-A for example. As shown in FIGS. 6 and 7, the ignition device has a male screw portion 22 formed on a housing 1 thereof to mount itself on an internal combustion engine. The male screw portion 22 is screw-fastened to a female screw portion 202 formed on a mounting hole 201 of a cylinder head 200 of the internal combustion engine.
The ignition device disclosed in U.S. Pat. No. 6,694,958-B, however, has an issue that a stress generated by mechanical vibrations of the internal combustion engine may snap the housing 1 a boundary between a fastened portion screw-fastened to the female screw portion 202 of the cylinder head 200 and a free portion. This issue is caused by a large distance between the screw-fastened portion and a barycenter of the ignition device and a large weight relative to a separate type spark plug.
Further, a clearance portion 13 is required at the boundary at an end of the male screw portion 11 of the housing 1 opposite to a combustion chamber of the internal combustion engine to form the male screw portion 11 on the housing 1. The clearance portion 13 has a diameter smaller than a diameter of the outer diameter of the male screw portion 11, and the ignition device may snap at the clearance portion 13 by the vibration of the internal combustion engine.